The bag states how much concrete it will produce in the units of cubic feet so you should first convert all your measurements to feet. There are 12 inches in a foot so
20 inches is 20/12 = 1.67 feet
and 3 inches is 3/12 = 0.25 feet
Thus the volume of concrete you need is
26 1.67 0.25 = 10.8 cubic feet.
One bag will produce 0.41 cubic feet so you need 10.8/0.41 = 26.3 bags. Hence you will need 27 bags.